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Threshold of discovery

Students design five-module, double-curved tensegrity structure using new thinking and technologies at Pratt Institute.

By Marion Hammon

Most architectural projects begin with a design problem, but Mark Parsons, visual artist and director of production technologies for the School of Architecture at Pratt Institute in Brooklyn, N.Y., took a different approach with the 10 students in his “Frame and Membrane” seminar recently. He presented them not with a design problem, but with design possibilities.

“We looked at what governs the relationship between material, process and resulting form,” Parsons says. “The structure the students ultimately created is an evolution of their focus on discovering what’s possible.”

Central to that evolution is an understanding of materials. Parsons explains, “An architect envisions a definition of space and qualities of environment, and achieving that vision requires taking into account the intrinsic capabilities of building materials—so if it’s a well-crafted spatial concept, the material wants to perform the way the architect imagined it. The students did an amazing job balancing special-material factors.”

The students’ individual research and experimentation led to the collaborative project Ethereal Threshold, a five-module, double-curved tensegrity structure with a dynamic surface that catches light and shadow in a way that is very expressive of its form.

Milliskin fabric and the computer numerical controlled (CNC) fabrication process were donated by industry sponsor Sperry Sails Inc. of Marion, Mass., continuing a longstanding relationship that began in 2009 with Light Canopy, another project created in one of Parsons’ design-build seminars at Pratt Institute.

Last winter, Ethereal Threshold was installed outside the Robert F. Siegel Gallery in the lobby stairway of the Steven Holl-designed wing of Pratt’s architecture building, Higgins Hall.

Thinking outside the box

Innovation through exploration is the process that guides Parsons’ seminars. The students began by testing their initial research and design ideas through computer modeling and physical prototyping. To do this, they used a number of design tools available at Pratt, including 3D modeling software, laser cutters, 3D printing and CNC.

They found enormous potential in a tensegrity module design that had the ability to scale up without losing the qualities of smaller models, and made it the focus of their project. The basic shape of the module resembled the entrance to a Shinto shrine, which hinted at the spiritual and suggested a space of transformation—a threshold.

Building a full-scale prototype of the module revealed another transformation, one that wasn’t evident from tabletop studies. When light hit the Milliskin fabric from the outside, it created opacity that helped articulate the structure’s curved surface, but when viewed from underneath the structure, the fabric allowed light to come through, highlighting the material’s translucency and adding to the ethereal quality the students were looking to create.

The design held an additional surprise related to the fabric. While the undulating surfaces and swooping curves on the outside suggested a complex structure on the inside, the interior was, in fact, clean and uncluttered. The key to this positive disconnect was that the fabric was not merely sheathing for the structure—through a “balanced tension” it played a significant role in the shape of the exterior.

The frame also offered a touch of the unexpected. The students combined steel, which successfully distributed the vertical force of the tensile structure, with aluminum, which maintained the shape of the double-curved surface while being light enough not to compromise the geometric integrity of the construction. Because the structural members don’t touch each other in the tensegrity condition, it was possible to mix metals without risk of corrosion, adding a distinctive and elegant nuance to the design.

Arriving at the threshold

To explore configurations and find a site for their modular threshold, the team studied transitions between spaces and tested the prototype module at a number of campus locations, including interior doorways and outdoor sites.

The beautifully illuminated, curved central stairway leading from the Higgins Hall lobby to the auditorium was chosen as the optimal site for Ethereal Threshold. In this prominent space, it could also be seen by the hundreds of architecture students and practicing faculty who make up Pratt’s architecture community on campus.

The location not only provided a dramatic journey between spaces, but also presented the team with additional design challenges necessary to accommodate the curve and descent of the stairway. They mapped the space and calculated the geometry to address these challenges, creating a five-module design that would offer the best views while moving through the Threshold and have the support necessary to uphold the structure.

Building the ethereal

Ethereal Threshold was now ready for construction, but there was nothing ethereal about the fabrication process. Because precision was essential at this stage, Sperry used a 50-foot-long CNC fabric cutter so that the shift in geometry of the fabric pieces would be executed correctly and the edges would meet properly.

“Sperry’s sponsorship helped my students execute their design at full-scale with professional results that would not have been possible otherwise,” says Parsons, whose fabric-oriented studios have also been sponsored by Hunter Douglas, Designtex and Fabric Images. “Working with industry partners helps ground students in real-world experience and produces new R&D for all participants using innovative materials and techniques.”

To assemble the individual modules and connect the Milliskin pieces, the students handled the stitching process together as a team, using a zigzag stitch that maintained the stretchiness of the fabric. They worked out the connection details and tieback points to keep the structure vertical without calling unnecessary attention to structural end points.

Because the complexity of Ethereal Threshold was front-loaded into the design, assembling it in the space was easy. The basic installation took only a few hours, with several more hours needed to make adjustments and streamline the appearance of the tieback points.

Exploring new options

For most of the students, the project was their first experience seeing their work move from idea through design to full-scale construction. The design and prototyping phases of Ethereal Threshold allowed them to gain a deeper understanding of the connection between material and form. It gave them insights into the architectural design possibilities of working with specialty fabrics that they will later take with them into professional practice.

For future projects, Parsons is interested in exterior fabrics that have a strong stretch coefficient and are tensile, fire-resistant, UV-resistant, and waterproof. “I’m really excited by the potential of this new category of exterior structural fabrics and in ways to apply them to architectural structures,” he explains.

This semester, Parsons’ students are using principles of tensile architecture to design rooftop enclosures for concrete buildings in the developing world. Says Parsons, “Fabric structures can provide the answer to a lot of real-world architectural issues, and do it beautifully.”

Marion Hammon, senior editorial manager at Pratt Institute, is a writer and editor who lives in New York City.